Hydraulic Water Redistribution by Silver Fir (Abies Alba Mill.) Occurring Under Severe Soil Drought

Article Hydraulic Water Redistribution by Silver Fir (Abies alba Mill.) Occurring under Severe Soil Drought 1,2, , 3, 1 1 Paul Töchterle y * , Fengli Yang y, Stephanie Rehschuh , Romy Rehschuh , Nadine K. Ruehr 1, Heinz Rennenberg 3 and Michael Dannenmann 1 1 Institute of Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), Karlsruhe Institute of Technology (KIT), Kreuzeckbahnstrasse 19, 82467 Garmisch-Partenkirchen, Germany; [email protected] (S.R.); [email protected] (R.R.); [email protected] (N.K.R.); [email protected] (M.D.) 2 Institute of Geology, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria 3 Institut für Forstbotanik und Baumphysiologie, Freiburg University, Georges-Köhler Allee 53/54, 79085 Freiburg, Germany; [email protected] (F.Y.); [email protected] (H.R.) * Correspondence: [email protected] These authors contributed equally to this study. y Received: 20 December 2019; Accepted: 29 January 2020; Published: 31 January 2020 Abstract: Hydraulic redistribution (HR) of water from wet- to dry-soil zones is suggested as an important process in the resilience of forest ecosystems to drought stress in semiarid and tropical climates. Scenarios of future climate change predict an increase of severe drought conditions in temperate climate regions. This implies the need for adaptations of locally managed forest systems, such as European beech (Fagus sylvatica L.) monocultures, for instance, through the admixing of deep-rooting silver fir (Abies alba Mill.). We designed a stable-isotope-based split-root experiment under controlled conditions to test whether silver fir seedlings could perform HR and therefore reduce drought stress in neighboring beech seedlings. Our results showed that HR by silver fir does occur, but with a delayed onset of three weeks after isotopic labelling with 2H O(δ2H +6000%), 2 ≈ and at low rates. On average, 0.2% of added 2H excess could be recovered via HR. Fir roots released water under dry-soil conditions that caused some European beech seedlings to permanently wilt. On the basis of these results, we concluded that HR by silver fir does occur, but the potential for mitigating drought stress in beech is limited. Admixing silver fir into beech stands as a climate change adaptation strategy needs to be assessed in field studies with sufficient monitoring time. Keywords: hydraulic redistribution; drought; silver fir; European beech; mixed stand 1. Introduction Hydraulic redistribution (HR) is the passive flux of water between wet- and dry-soil zones through plant roots as conduits. It is driven by soil-water potential gradients between dry- and wet-soil layers, and between roots and soil matrix [1,2]. Typically, HR occurs during the night, when transpiration has ceased [3–5]. Water can be redistributed in the upward (i.e., hydraulic lift [2,6,7]), downward (i.e., hydraulic descent [8–11]), and lateral directions [12–15]. Field observations showed that HR plays an important role in terrestrial ecohydrological cycles. Plants can benefit from HR through enhanced photosynthesis and transpiration [16], alleviated soil-moisture loss during the dry season [17], and a prolonged growing season [18,19]. These immediate benefits of HR consequently enhance nutrient acquisition [20], increase nutrient mobility, and facilitate root-litter decomposition [21,22]. Forests 2020, 11, 162; doi:10.3390/f11020162 www.mdpi.com/journal/forests Forests 2020, 11, 162 2 of 12 HR has been documented in more than 100 species [23], including agricultural crops and grasses [24–26], and forest trees and shrubs [27–29]. Although HR has been observed in diverse climatic settings [16,23,30,31], it is most prevalent in arid and semiarid ecosystems, such as savannas [16,32,33], arid climates and semideserts [5,34–36], Mediterranean-type ecosystems [10,37–39], and tropical forests [40–42]. However, HR may become increasingly relevant in temperate ecosystems that are subject to intensified drying–wetting cycles due to extreme drought events in projected climate-change scenarios [27,43]. Several techniques can be used to identify HR under laboratory or field conditions. Reverse water flow in roots can be identified by sap-flow techniques like the heat-balance [29,44] or -ratio method [45,46]. However, quantification of reverse water flow in roots by sap-flow measurements can easily be disrupted by fluctuations in ambient temperature [46,47], or small-scale soil and geomorphic variability [10]. Furthermore, sap flow can only be measured in individual roots and upscaling to the root system is a significant source of error [48]. Contrastingly, measurements of soil-water potential near plant roots, accompanied by water stable isotope analyses, have been used to track water movement in soil [30,49,50]. In this context, stable isotopes, either at natural abundance or by using heavy isotope 18 2 enriched water (i.e., H2 O, H2O) as a tracer, are used as a novel technique to quantify water flow between dry- and wet-soil layers through plant roots and water uptake by adjacent plants [3,27,51]. In temperate forests, HR was only detected in a few tree species in the field, such as Norway spruce (Picea abies (L.) Karst.), Douglas fir (Pseudotsuga menziesii), ponderosa pine (Pinus Ponderosa), loblolly pine (Pinus taeda), and sessile oak (Quercus petraea)[51–55]. In an adult mixed oak/European beech forest, Zapater et al. [51] showed HR by oaks using an 18O-labelling approach but did not find any tracer material in European beech. However, both HR and the uptake of redistributed water by neighboring plants was detected in studies with seedlings of English oak, Norway spruce, and European beech under moderate drought in split-root systems in the greenhouse [27]. In Central Europe, European beech, being both an abundant natural tree species and a key species in forestry, was reported to be particularly vulnerable to drought [56,57]. As extreme drought events and intensified drying–wetting cycles are projected to become more prevalent [43,58,59], beech forests in Central Europe face consequences such as declining growth and drastic economic losses for forestry [60–62]. Admixing deep-rooting tree species could potentially increase the resilience of beech stands. In this context, silver fir was proposed due to its high productivity and presumably higher drought resistance [63,64]. Furthermore, recent studies indicate that water supply to European beech in mixed forest stands may be supported by the presence of silver-fir neighbors [65,66]. The aim of this study was to show if silver fir can perform HR under extreme drought conditions. For this purpose, we applied an improved split-root approach under controlled conditions, combined 2 with the H2O labelling of water and in situ stable isotope analysis of soil moisture. We hypothesized 2 that fir roots were able to allocate H2O from moist- to dry-soil zones by HR. 2. Materials and Methods 2.1. Mesocosm Setup This study was conducted using plant-soil mesocosms under controlled conditions in the scientific greenhouse at the KIT Campus Alpin in Garmisch-Partenkirchen, Germany. Temperature (T) and relative humidity (rH) were controlled and underwent daily cycles (T = 20.5 4 C, and rH = 58 ± ◦ ± 12 % on average). Ambient CO2 concentration showed diurnal fluctuations between 380 and 450 ppm without long-term trends during the timespan of the experiment. Six mesocosms, each comprising 2 nested polyvinylchloride (PVC) compartments (dimensions of inner and outer compartment: length width height = 90 38 40 cm3 and 20 20 10 cm3, × × × × × × respectively) were set up as shown in Figures1 and2. Soil for the mesocosms was collected in autumn 2015 in the Black Forest close to Emmendingen (SW Germany). The material was taken from the Ah horizon of a Dystric Cambisol that originated from Triassic sandstone and showed a sandy loam texture Forests 2020, 11, 162 3 of 12 texture (see [67] for site and soil details). The soil material was mixed with perlite at a volume ratio of 1:1. Perlite is a highly porous mineral that improves soil drainage and aeration properties while retaining moisture. These properties helped with the homogenization of soil moisture and isotope equilibration. Two silver-fir seedlings (3.5 years of age) were planted in the outer compartment (hereafter referred to as the fir compartment), and a single European beech seedling (2 years of age) in the inner compartment (hereafter referred to as the beech compartment). Root length of the fir and beech Forests 2020, 11, 162 3 of 12 Forestsseedlings 2020, 11 was, 162, on average, 30 and 15 cm, respectively. A first-order coarse root with intact second5 of 12- and third order fine roots of each silver fir seedling was redirected to the beech compartment. The A Picarro L 2130-i cavity ringdown spectrometer was used to analyze the stable isotopic (seemesocosms [67] for sitewere and then soil wrapped details). with The plastic soil material film to avoid was mixedevaporation with perliteand consequent at a volume contamination ratio of 1:1. composition (δ2H) of the gas stream. On measurement days, gas streams exiting each of the 6 Perliteof the islab a highlyenvironment porous with mineral 2H2O that vapor improves. The fir soil-root drainage strand ands were aeration the only properties hydraulic while connection retaining mesocosms were continuously measured for at least 15 min. To minimize carryover artefacts, lines moisture.between Thesebeech propertiesand fir compartment helped withs. theAs homogenizationexperiment control, of soil 1 mesocosm moisture andwas isotope left unplanted equilibration. but were flushed for 20 min between each measurement, and the values for the first 5 min of each otherwise received identical treatment (Figure 1). measurement were ignored. Stable isotope data reported in the results section of this study refer to the average values of the remaining measurement time. Results of each day were scale-normalized to 2 in-house standards (δ2H = –235.0 ± 1.8 ‰ and 1.8 ± 0.9 ‰ VSMOW) that were calibrated against international reference materials (VSMOW2, SLAP2).

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